Spiking neural P systems with weights (WSN P systems, for short) are a new variant of spiking neural P systems, where the rules of a neuron are enabled when the potential of that neuron equals a given value. It is known that WSN P systems are universal by simulating register machines. However, in these universal systems, no bound is considered on the number of neurons and rules. In this work, a restricted variant of WSN P systems is considered, called simple WSN P systems, where each neuron has only one rule. The complexity parameter, the number of neurons, to construct a universal simple WSN P system is investigated. It is proved that there is a universal simple WSN P system with 48 neurons for computing functions; as generator of sets of numbers, there is an almost simple (that is, each neuron has only one rule except that one neuron has two rules) and universal WSN P system with 45 neurons.